The present invention relates to an optical range finder incorporated in a camera or the like to be used for automatic focus adjustment. More specifically, the invention relates to a technology for obtaining measurement accuracy by making distance determination multistage. The invention also relates to a multistage determination process or algorithm. In addition, the invention relates to a light quantity calibration technology.
FIG. 12 is a schematic perspective view showing an example of a camera incorporating a conventional optical range finder. A camera 25 is provided with a lens barrel 27 on a front of a body 26. The optical range finder composed of a projection unit 1 and a photoreceiving unit 2 is mounted to the body 26. The projection unit 1 projects optical flux to an object located in an optical axis direction. The photoreceiving unit 2 receives the optical flux returned from the object, and outputs a detection signal in accordance with a light quantity of the received optical flux. The optical range finder determines a distance to the object based on the detection signal outputted from the photoreceiving unit 2, and executes ranging by using a relation that a reflected light quantity from the object is inversely proportional to a square of the distance.
FIG. 13 is a schematic view showing a specific constitution of the conventional optical range finder. The projection unit 1 and the photoreceiving unit 2 are assembled into a holding frame 30. This holding frame 30 is supported on a circuit board 4a. The circuit board 4a is attached in the body 26 of the camera 25 shown in FIG. 12. The projection unit 1 is constituted of a projection lens 1b mounted to a front of the holding frame 30, and a light emitting element 1a for emitting optical flux such as infrared rays to the lens 1b. On the other hand, the photoreceiving unit 2 is constituted of a lens 2b mounted to the front of the holding frame 30 to converge the optical flux returned from the object, and a light detecting element 2a arranged oppositely to the lens 2b. The light detecting element 2a outputs a detection signal having a current amount in accordance with the received light quantity. A circuitry unit assembled in the circuit board 4a processes the detection signal to execute ranging.
The optical range finder of the light quantity type is simple in structure compared with a range finder based on a principle of triangulation. Accordingly, in the conventional optical range finder, a binary system for determining a distance in two stages, i.e., far and short distance zones, is often used. That is, the circuitry unit of the optical range finder is provided with a comparator for comparing the detection signal with a predetermined reference level to determine whether the object is located in a far distance side or a short distance side. As the optical range finder executes ranging based on a light quantity reflected from the object, accuracy is not so high because of factors described below, and it has been considered improper to increase accuracy from the conventional two-stage determination to multistage determination of three stages or more. As the light quantity depends on a reflectance of the object (target of the camera or the like), measurement accuracy cannot be obtained, and it has been considered meaningless to set multiple stages. In addition, circuitry needs only one comparator in the case of binary determination, and is very simple, and importance has been placed on practical rationality. For a request of setting of multiple stages, a system based on a normal triangular ranging system is suited, and there has been no point in setting multiple stages in the optical range finder of the light quantity type.
However, following recent diversification of cameras, low cost and miniaturization have been requested of the range finder used for automatic focus adjustment or the like. Thus, there has been a demand for setting multiple stages in the optical range finder of the light quantity type, which is advantageous in cost and size. The present invention proposes setting of multiple stages in the optical range finder of the light quantity type.
In the case of executing determination of the light quantity reflected from the object in multiple stages, a complicated system might be employed, which prepares a plurality of determination circuits in accordance with the number of stages, and a plurality of reference levels in accordance with a variable light quantity. Alternatively, means may be employed, which directly measures a level of a reflected light quantity and converts it into numerical value data, and calculates object distance information based on a physical relation between the distance and the light quantity. However, the above-described measures both result in complex circuitry, and thus they are not suitable for the optical range finder which should be simple ranging means. In a general purpose optical range finder, simple determination of far and near stages is executed, the number of reference levels is one, and a circuit is very simple. In the market, there is a demand for means making the optical range finder multistage without complicating circuitry, which is now a problem to be solved. In this case, a process or algorithm of multistage determination is also a problem to be solved. In addition, if the optical range finder of the light quantity type is made multistage, ranging accuracy becomes higher than the conventional bi-stage type and, accordingly, error factors must be suppressed. In such a case, it is considered effective to execute calibration of a light quantity of projected beam beforehand, which is also a problem to be solved.
In order to solve the foregoing problems of the prior art, first means has been contrived. That is, the present invention is directed to an optical range finder having a projection unit for projecting optical flux to an object, a photoreceiving unit for receiving the optical flux returned from the object and outputting a detection signal in accordance with a light quantity of the received optical flux, and a circuitry unit for conducting a measurement of a distance to the object based on the detection signal, wherein the circuitry unit comprises a light quantity determination circuit for comparing a signal level of the detection signal with a reference level to carry out binary determination of the light quantity of the returned optical flux, a reference level setting circuit for switchably supplying a plurality of reference levels to the light quantity determination circuit, the reference levels being preset in correspondence to multiple stages of the distance, and a control circuit for controlling the reference level setting circuit to supply the reference levels while switching the reference levels so as to enable the light quantity determination circuit to execute a plurality of the binary determination with the supplied reference levels for one measurement, thereby specifying the distance to one of the multiple stages based on the thus obtained plurality of the binary determination.
Preferably, the reference level setting circuit sets the reference levels in correspondence to the multiple stages of the distance where inverse numbers of the distance of the multiple stages are arranged in an arithmetic series. For example, the reference level setting circuit sets the reference levels in correspondence to the multiple stages of the distance where the distance is divided into the multiple stages based on a harmonic sequence (1/2, 1/3 . . . 1/n). Preferably, the reference level setting circuit comprises a plurality of current supplies arranged in parallel with each other to output different current amounts, the plurality of the current supplies being set so that the respective current amounts may be arranged in a geometric series where 2 is a base, and a switch for switchably combining the current supplies to generate a multiple of current amounts such that the multiple of the current amounts are supplied to the light quantity determination circuit as the reference levels. In such a case, the plurality of the current supplies comprise a single current source and a plurality of resistive elements connected to the single current source to define the plurality of the current supplies generating the respective current amounts, the resistive elements having resistance values set in the geometric series where 2 is a base. The plurality of the current supplies may include a current supply for outputting a minimum current amount appropriate to a reference level corresponding to the farthest stage of the distance. The plurality of the current supplies may comprise at least four current supplies where the respective current amounts are set to 1:2:4:8, and these four current supplies are combined to set the multiple of the current amounts appropriate for the reference levels corresponding to the stages of the distance. The four current supplies may include a current supply which outputs a minimum current amount and which is always combined to the remaining current supplies in providing any of the reference levels.
According to the present invention, by directly using the light quantity determination circuit constituted of only one comparator incorporated in the general purpose optical range finder, ranging can be made multistage. The plurality of reference levels are supplied in a switchable manner to the single light quantity determination comparator. When one ranging is executed, the reference levels are switched to execute a plurality of determinations and, based on the thus obtained results, an object distance is measured. For example, the plurality of reference levels set corresponding to the divided distance stages of three zones or more are sequentially supplied to the comparator to repeatedly execute binary light quantity determination. Then, when an output of the comparator is reversed, the distance stage corresponding to the reference level can be set as a ranging value.
Furthermore, second means has been contrived to solve the technical problems of the prior art. That is, the present invention is directed to an optical range finder having a projection unit for projecting optical flux to an object, a photoreceiving unit for receiving the optical flux returned from the object and outputting a detection signal in accordance with a light quantity of the received optical flux, and a circuitry unit for conducting a measurement of a distance to the object based on the detection signal, wherein the circuitry unit comprises a light quantity determination circuit for comparing a signal level of the detection signal with a reference level to effect a binary determination of the light quantity of the returned optical flux, a reference level setting circuit for switchably supplying a plurality of reference levels to the light quantity determination circuit, the reference levels being preset in correspondence to multiple stages of the distance, and a control circuit for controlling the reference level setting circuit to sequentially supply the reference levels based on an algorithm while switching the reference levels so as to enable the light quantity determination circuit to sequentially execute sessions of the binary determination with the supplied reference levels for one measurement, thereby specifying the distance to one of the multiple stages according to the algorithm based on the sessions of the binary determination, where the algorithm is designed to specify the distance to one of a far zone and a near zone over the multiple stages of the distance at an initial session of the binary determination, then specify the distance to one of a far sub-zone and a near sub-zone within the specified one of the far zone and the near zone at a next session of the binary determination, and specify the distance to one of the multiple stages within the specified one of the far sub-zone and the near sub-zone at a further session of the binary determination.
Preferably, the algorithm is designed to enable the light quantity determination circuit to repeat a comparison of the signal level of the detection signal with the reference level and to count the results of the comparison in terms of a far side and a near side, such that one session of the binary determination of the light quantity is finished when the count of either of the far side and the near side reaches a predetermined number. Further, the algorithm is designed to enable the light quantity determination circuit to settle a current session of the binary determination in reverse to a previous session of the binary determination when a result of the comparison reverse to those results of the comparison involved in the previous session appears in the current session of the binary determination. Moreover, the algorithm is designed to enable the light quantity determination circuit to make a next session of the binary determination final when a current session of the binary determination involves split results of the comparison splitting between the far side and the near side, thereby specifying the distance to one of the multiple stages at the next session of the binary determination.
According to the present invention, by directly using the light quantity determination circuit constituted of only one comparator incorporated in the general purpose optical range finder, ranging can be made multistage. The plurality of reference levels are supplied in a switchable manner to the single light quantity determination comparator. When one ranging is executed, the reference levels are switched to execute a plurality of determinations and, based on the thus obtained results, an object distance is measured. In a conventional optical ranging, a determination level is set to one point, and far and near binary determination is executed for the set level. A ranging zone at this time is wide. On the other hand, according to the present invention, determination levels are prepared in multiple stages, the levels are switched to execute multi-stage determination, and each distance zone is narrowed, whereby ranging accuracy is increased. In this case, because of the need to execute a number of binary determinations by switching the levels, ranging time is extended. On the other hand, since intervals of determination zones are narrowed, a required level of determination accuracy becomes higher compared with that of the conventional case. Under these circumstances, according to the present invention, ranging time and determination accuracy are balanced, and then a rational determination algorithm is used. That is, in execution of multistage determination, a process is beforehand programmed to properly switch determination levels to realize shortest measurement. Specifically, determination level setting is placed at an intermediate stage of the determination zones, and the process further proceeds to a sub-divided intermediate stage in the selected zone. In this way, compared with the case of sequentially scanning up or down the determination levels, the total number of determinations can be reduced. Moreover, an algorithm is employed, which sets a present determination result in accordance with a previous determination result to thereby minimize the number of determinations.
In addition, third means has been contrived to solve the technical problems of the prior art. That is, the present invention is directed to an optical range finder having a projection unit including a light emitting element for projecting optical flux to an object, a light quantity adjusting unit for provisionally adjusting a light quantity of the projected optical flux, a photoreceiving unit for receiving the optical flux returned from the object and outputting a detection signal in accordance with a light quantity of the received optical flux, and a circuitry unit for conducting a measurement of a distance to the object based on the detection signal, wherein the circuitry unit comprises: a light quantity determination circuit for comparing a signal level of the detection signal with a reference level to carry out binary determination of the light quantity of the returned optical flux; a reference level setting circuit for switchably supplying a plurality of reference levels to the light quantity determination circuit, the reference levels being preset in correspondence to multiple stages of the distance, and a control circuit for controlling the reference level setting circuit to supply the reference levels while switching the reference levels so as to enable the light quantity determination circuit to execute a plurality of the binary determination with the supplied reference levels for one measurement, thereby specifying the distance to one of the multiple stages based on the thus obtained plurality of the binary determination, and wherein the light quantity adjustment unit comprises a group of parallel resistive elements which are selectively connected in parallel to the light emitting element for forming bypasses of electric currents flowing through the light emitting element to thereby execute a primary adjustment of the light quantity, and another group of serial resistive elements which are selectively connected in series to the light emitting element for regulating the electric current flowing through the light emitting element to thereby execute a secondary adjustment of the light quantity.
Preferably, the group of the serial resistive elements are set so that respective resistance values of the serial resistive elements may be arranged in a geometric series where 2 is a base. Preferably, the light emitting element is driven under constant voltage control. Otherwise, the light emitting element is driven under constant current control.
According to the present invention, by directly using the light quantity determination circuit constituted of only one comparator incorporated in the general purpose optical range finder, ranging can be made multistage. The plurality of reference levels are supplied in a switchable manner to the single light quantity determination comparator. When one ranging is executed, the reference levels are switched to execute a plurality of determinations and, based on the thus obtained results, an object distance is measured. For example, the plurality of reference levels set corresponding to the divided distance stages such as three stages or more are sequentially supplied to the comparator to repeatedly execute light quantity determination. Then, when an output of the comparator is reversed, the distance stage corresponding to the reference level can be set as a ranging value.
Incidentally, in the conventional light quantity ranging, far and near binary determination is executed where a determination level is one stage, distance setting zone of each of far and near stages is wide, and a required level of determination accuracy is low. However, if a determination level is made multistage according to the present invention as described above, since a distance range for one stage is narrowed, higher determination level accuracy is required.
In the light quantity ranging, various error factors are involved because an absolute value of the light quantity is set as a signal. For example, variances in light emission degrees of components such as transmittances of the light emitting element, the light detecting element, and the lens, determination level setting, and processing circuits for executing ranging have been error factors.
As described above, variance factors of the light quantity signal are wide-ranging, and a variance width is large. Normally, the error factors may be dealt with by adjusting determination level setting. However, if determination steps are set for multistage ranging corresponding to a relative light quantity as in the case of the present invention, adjustment must be executed in multiple stages, and therefore this method cannot be employed. Thus, absolute value adjustment of the light quantity must be executed by another function, which requires an adjustment method of wide-ranging calibration.
Thus, according to the present invention, the light quantity is adjusted by a drive current of the light emitting element such as an infrared-emitting diode (IRED). In the case of multistage ranging, adjustment accuracy of the drive current can be set sufficiently small with respect to a relative light quantity step and, in order to permit and absorb variance in various components. Thus, according to the present invention, a group of parallel resistive elements is used to enable primary adjustment, and another group of serial resistive elements is used to enable secondary adjustment. The group of parallel resistive elements is connected in parallel to the light emitting element so as to be selected, a bypass of the drive current flowing through the light emitting element is formed to enable level shifting, whereby an adjustment width is enlarged. On the other hand, the group of serial resistive elements is serially connected to the light emitting element selectively, and combined resistance is finely set to regulate the current flowing through the light emitting element at fine levels.